Harvest under dry climates: High Sugar vs. High Color.

Sugar-color

In this blog we discuss why adapting harvest date to a warming environment is challenging …and why you may not have to trade off sugar concentration to get high color concentration.

Higher sugar concentrations at harvest are often associated with an increase in temperature. This is observed empirically and scientifically in the last decades throughout many wine regions. But, if climate is warming, why not simply harvesting systematically earlier? That way, fruit would be exposed to higher temperatures for a shorter period of time and maybe the amount of sugar would not rise as much?

Why harvesting with a high sugar concentration?

Australian researchers have demonstrated that, even small increases of berry temperatures uncouple the color content and sugar content with red varietals. It also affects other berry sensory traits, like aromas. To compensate for the delay in color accumulation versus sugar accumulation, winemakers have 2 options:

  1. Harvest according to sugar level. It means fruit color will be less than what it could be. Flavor will probably be on the vegetative side. Color incorporation into tannins will also be less.
  2. Harvest according to color accumulation. It means harvest decision will track color variations and not so much focus on sugars.

Isn’t sugar accumulating continuously?

The simple answer is “No”. We must dissipate this false belief.

Under arid climate (meaning no rain near harvest) it is not correct to think that: “the more you delay harvest, the more sugar accumulates”. Instead, the correct sentence is: “the more you delay harvest, the more sugar is susceptible to concentrate due to water loss”. Discussing the difference between berry sugar accumulation and concentration is beyond the scope of this blog. However, it has already been discussed here.

Conclusions

If you observe more elevated temperatures during ripening, then, you know that color concentration will reach a peak according to a time profile which does not track sugar accumulation. Technically, winemakers will say that colors and sugars are no longer synchronized. It means sugar accumulation is not “coupled” with color accumulation…and this should become the “fruit ripening norm” if warm vineyards are getting warmer. Thus, if you ask a “dry climate” winemaker or if you are one, it will not be a secret. Many good wines can be made from grapes reaching more than 28 brix (i.e. potential alcohol higher than 17%)…However, that does not mean that sugar accumulated continuously until harvest disrupted the process. How do we know that?

There is a limit to the maximal  amount of sugar per berry

If you are a dry climate winemaker, and if you select option 2 for harvest, here is what often happens next.  Postponing harvest leads to high fruit sugar concentration. This results in wines with high alcohol content after fermentation, which can be hard to sell.

But here is the good news: if you understand the processes driving sugar accumulation, you are not doomed to harvest fruit at 28 Brix or higher to get more color and more complete ripening. Research has shed light on chemical changes observed in ripening grape berries and you can use it to your advantage. If you understand what drives berry susceptibility to concentrate sugar, then – with the right practices- you can keep a low brix with a higher color content. All it takes is understanding microclimate and vine transpiration effects on berry shriveling susceptibility. Time profile of sugar concentration is the result of three phenomenons occurring simultaneously at the berry level:

  1. Sugar import,
  2. Sugar metabolism,
  3. Changes in water quantity.

From this complex interplay, the ripening growth period is divided into 2 phases :

  • Phase A:  Sugars are unloading from phloem until berry sugars concentrations reach values near 1.1 M (source). During that phase sugar import is 5 to 10 times more important than sugar metabolism or water budget variations (source).
  • Phase B:  Sugars susceptibility to concentrate by berry shriveling is due to evapotranspiration.

How is that useful to reduce brix near harvest?

Assuming you will not harvest during phase A (color is too low, flavors are not ripe), then you should focus on what causes sugar concentrations to increase during phase B if you want to harvest at a lower brix. During phase B,  sugar import has stopped. Your attention is thus solely on berry sugar metabolism and water budget. And here again the researchers have found that water shortage becomes the main factor responsible for increasing sugar concentration during phase B. Water shortage lowers berry water quantity and sugar concentration increases as a result.

In simple terms: water shortage is the culprit driving your brix to higher values during phase B. In practice you should minimize anything causing a water shortage during phase B if you want to keep your brix low.

The last question is: what causes a berry water shortage during phase B? At this point you are left with 3 main drivers: temperature and humidity within the cluster zone and the amount of water the vine can provide to the berry until harvest day.

Ultimately, controlling those 3 drivers until the day of harvest is key to reach a full maturity while keeping the brix lower, even under warmer conditions.


TAKE HOME:

If you are a dry climate winemaker, make wine in the vineyard :

  • Exert a tighter control over evapotranspiration conditions during phase B. By monitoring and acting on microclimate variations and vine water flow to the berry, you can maintain a low brix until harvest.
  • Use alternatives instead of reducing wine alcohol content in the cellar. Manipulating microclimate and irrigating pre harvest is an essential part of winemaking operations. If climate keeps warming up in dry areas, this will be soon your best option. It saves on cost and time in cellar operations. But, more importantly,  it will be your only way to harvest a mature grape which is not also a sugar bomb.

Coming next:

Winemakers often postpone harvest weeks after maximal sugar content per berry has been reached, particularly under semi arid climates. One of the rewards is to get more color extraction from the berries. But why would that be important?….

in a future blog, we will discuss  why “Color is not only useful for color” … and why delaying harvest in hope to gain more color also affects other wine sensorial properties…stay tuned!

Fruition Sciences

At Fruition Sciences, we acknowledge uniqueness, natural abilities and potential growth for plants and people. While respecting tradition, we provide winemakers with a highly integrated, terroir and vintage specific, data driven web application.

Follow us on social networks :
Facebooktwittergoogle_pluslinkedinyoutube

Share this post :
Facebooktwittergoogle_pluspinterestlinkedinmail

3 thoughts on “Harvest under dry climates: High Sugar vs. High Color.

  1. I read this post early this morning and almost didn’t comment. Based on my experience in a semi-arid climate this line of thought is too simplistic for what can actually be achieved under full water control to the vine. Hanging grapes on the vine for long periods of time result in the loss of novel aromatics and lessen the ability for the winemaker to control extraction.

    Increasing color and physiological ripeness is a function of stress management in the vineyard. Irrigation being the number one factor in achieving this. But, what we are really trying to affect is ABA increase pre-veraison into the skins which affect the physiological ripening post-veraison.

    Under a system that achieves as much stress as possible without knocking any leaves on the vine during the season we can maximize each seasons berry concentration. In part by reducing berry size and in part by influencing physiological ripeness. Most of the winemakers I have been working with manage down to 2200-2500 mg/L of Total Polyphenols while harvesting in many cases 1-2 weeks before other vineyards in the area. This is independent of heat accumulation and soil variations from one vineyard to the next.

    1. Hello Andrew,

      Thank you for sharing your insights and initiating a conversation. The goal of this blog is not to say what’s right or wrong but to communicate on recent findings. Harvest decision is an extremely complex process and we are obviously looking at it from a simplistic eye at first. In this blog, we simply focus on sugar and color. In the future, we’d like to share with the community thoughts about other factors such as : color extraction and its effect on tannin extraction, effects of sugar/acid ratio variations on astringency, relationship between astringency vs. bitterness, flavor profile variations in response to cluster temperature, effect of disease exposure, influence of personal wine style, etc. …with such conversations, we would like to discuss to which extend harvest date plays a role in that process. Last, in the context of warmer temperatures, we’d like to start a conversation on how higher temperatures will affect overall balance between all those complex parameters

      Regarding ABA: as reported in (http://www.ajevonline.org/content/67/3/356) it is a fact that modulating the severity of water deficit pre veraison will directly impact the onset of color pigmentation. As such, exerting a tighter control over Water deficit severity pre-veraison will affect the time gap between peak color and peak sugar on a per berry basis. However, considering the complex interactions between seasonal climatic conditions and vine water status regulation mechanisms (i.e. through its effect on transpiring leaf area development, root area growth, Nitrogen uptake, etc…) it is difficult to know how much ABA is being synthesized as soil moisture availability is gradually more limited at the root absorption sites. This being said, as you are pointing out, using vine based measurements to control vine water use is a powerful tool to speed up or slow down the rate of sugar and color accumulations. I suggest another very interesting article on the topic of ABA vs ripening events (https://www.ncbi.nlm.nih.gov/pubmed/26590311).

      When you mention 2200-2500 mg/L of total polyphenols, would you mind saying what is the extraction technique you are using? Also how much do you think the level of berry cell walls degradation may contribute to reaching that level of polyphenol extraction? The thinking here is : if your irrigation management hastens the onset of senescence (due to moderate water deficit imposed pre veraison), you may also increase the ease with which polyphenolic compounds are released early during maceration. The method of extraction and the effect of anthocyanin at boosting tannin extraction may play a great role in achieving those concentrations of total polyphenols. It would be interesting to analyze the relative effects of high temperatures vs early senescence on polyphenols extractibility. I hope this is helpful. Kind regards

      1. Thanks for sharing these observations.
        The sugar accumulation, sugar total, and sugar concentration are an interesting set of interactions. I’ve observed Brix hit 23.8 then drop to 20.2.
        Up ,down, up again.
        In a very arid climate, I’ve learned to stress a bit post bloom to minimize cell per berry. Then from verasion provide adequate water to keep leaves green and vines content. Trying to have no new vegetative growth after August 15. Gotta have some benchmark.
        We pick at 22.5-23.5 Brix. We don’t discriminate on the basis of color or cultivar.
        We find plenty of “color” in our gold and ruby wines, and wonderful aromatics.
        As minimalists we like pH below 3.5. As folks who drink wine with meals we like some acidity. Perhaps if we relied on critics, competitions, and old fat guys who smoke cigars we would make late harvest Cabernet Sauvignon.
        Paul Vandenberg
        Paradisos del Sol
        Zero Pesticide ingredient labeled wines.

Leave a Reply

Your email address will not be published. Required fields are marked *